SYDNEY, Sept 1 (Reuters) - Chinese rebar steel futures rose by as much as 6 percent on Friday, the biggest one-day gain since April 2016, buoyed by further signs of strength in Chinese industrial activity.

China’s manufacturing activity expanded at the fastest pace in six months in August, buoyed by a surge in export orders and higher prices, according to the private Caixin manufacturing purchasing managers index.

A day earlier, the official Purchasing Managers’ Index (PMI) rose to 51.7 in August from 51.4 a month earlier.

The positive data also boosted Chinese iron ore futures and was seen supporting U.S. dollar-denominated iron ore prices.

The most active rebar on the Shanghai Futures Exchange was trading up 5.7 percent at 4,070 yuan ($618.29) a tonne by 0600 GMT. The construction steel product was at its highest price since early 2013.

Iron ore for January delivery on the Dalian Commodity Exchange was up more than 4 percent, heading back towards five-month highs hit last week.

Benefiting from fatter margins, Chinese steel producers have boosted steel production and restocked on raw materials, prompting investment bank Macquarie to upgrade its iron ore price forecast for the third quarter to $73 from $50.

Iron ore for delivery to China’s Qingdao port .IO62-CNO=MB stood at $78.91 a tonne, up 3.3 percent from the previous quote, according to Metal Bulletin. ($1 = 6.5827 Chinese yuan renminbi) (Reporting by James Regan; Editing by Richard Pullin)